1. Statement of the Technical Field
The invention concerns communication systems. More particularly, the invention concerns communications systems implementing methods for frame synchronization.
2. Description of the Related Art
There are many conventional digital communication systems known in the art. The digital communication systems typically comprise communication devices (e.g., radios) and Fixed Network Equipment (“FNE”). The communication devices are each communicatively coupled to the FNE via a Radio Frequency (“RF”) link. The RF link is established by sending a digitally modulated signal from a communication device and receiving the digitally modulated signal at the FNE.
The digital communication systems often employ Time Division Multiplexing (“TDM”). TDM is a type of digital multiplexing in which two or more signals or data streams are transferred simultaneously as sub-channels in one communication channel. The communication channel is divided into timeslots for each of the sub-channels. During operation, data for a first sub-channel is transmitted during odd numbered timeslots (e.g., a first timeslot and a third timeslot). Data for a second sub-channel is transmitted during even numbered timeslots (e.g., a second timeslot and a fourth timeslot).
In these digital communication systems, one or more short transmission data bursts are assigned to each timeslot. A data burst typically consists of various types of information. For example, as shown in FIG. 1, an inbound 4V data burst 100 of a Project 25 (“P25”) Phase 2 Time Division Multiple Access (“TDMA”) based communication system consists of ramp guard information 102, 120, pilot symbols 104, 118, four (4) voice frames 106, 108, 114, 116, Encryption Synchronization Signaling (“ESS”) information 112 and Data Unit Identifiers (“DUIDs”) 110. In contrast, as shown in FIG. 2, an inbound 2V data burst 200 of a P25 Phase 2 TDMA based communication system consists of ramp guard information 202, 218, pilot symbols 204, 216, DUIDs 208, two (2) voice frames 206, 210 and ESS information 212, 214. The ramp guard information is used to prevent overlap between transmitting frame timing and receiving frame timing. The voice frames include digital voice data.
Before the processing of the data bursts begins, the FNE needs to synchronize symbol timing and frame timing with the transmitting communication device. Symbol synchronization is performed to find a correct down sampling instant. Frame synchronization is performed to determine the structure of a frame, i.e., to identify the location of frames within a voice burst sequence. The synchronization is performed to ensure that the FNE knows exactly where a data stream begins in a received RF signal and where the start/end of a TDM frame is within the data stream. The synchronization is typically achieved using a known synchronization pattern. The synchronization pattern is transmitted in an RF signal from the communication device prior to the transmission of data bursts. At the FNE, the synchronization pattern is located within the received RF signal. Once the synchronization pattern has been located, the FNE performs operations to establish symbol and frame synchronization with the communication device. Although the utilization of a known pattern enables symbol and frame synchronization between the communication device and FNE, it results in an inefficient utilization of bandwidth since the synchronization pattern does not include any user and/or control data.
The conventional digital communication systems also typically employ a late call entry mechanism for the FNE that misses a normal call entry, i.e., that misses the known synchronization pattern. For example, a P25 Phase 2 TDMA based communication system employs a technique in which Slow Associated Control CHannel (“SACCH”) information is used to facilitate timing synchronization between a communication device and an FNE which is trying to enter a call late. This SACCH technique will be described below in relation to FIG. 3.
As shown in FIG. 3, inbound communications 300 include a known synchronization pattern 302 and at least two ultraframes 350. The phrase “inbound communication” as used here refers to a communication transmitted from a transmitting communication device to the FNE. The known synchronization pattern 302 can be included in one or more Media Access Control (“MAC”) Push-To-Talk (“PTT”) Protocol Data Units (“PDUs”), which are not shown in FIG. 3. A first ultraframe 350 includes superframe data 304, 308, 312, 316, inbound talker SACCH data 306, 310, 314 and listener SACCH data 318. A second ultraframe 350 includes superframe data 320, inbound talker SACCH data 322 and other data (not shown in FIG. 3). The superframe data 304, 308, 312, 320 includes voice data that is being communicated from a first communication device to the FNE. The SACCH data 306, 310, 314, 322 includes synchronization information communicated from the first communication device to the FNE that is useful for facilitating late call entry. The superframe data 316 includes data (e.g., a request to join an existing call) transmitted from a second communication device to the FNE. The SACCH data 318 includes call specific information communicated from the second communication device to the FNE.
As noted above the synchronization pattern 302 is used to establish frame synchronization between a transmitting communication device and the FNE during a normal call entry. Once the synchronization has been established via the synchronization pattern 302, the FNE does not need to perform additional frame synchronization operations for subsequently received TDM/TDMA frames. Still, the FNE may monitor the inbound talker SACCH data to ensure that the frame synchronization is maintained.
However, if the FNE misses the synchronization pattern 302 and/or MAC PTT PDU(s), then the FNE has no idea where the voice data is in a received RF signal. As such, it must perform a late call entry process to establish frame synchronization with the transmitting communication device. The late call entry process involves relying on synchronization information contained in inbound talker SACCH data. In this regard, the FNE must wait for an inbound talker SACCH data (e.g., SACCH data 306) and use synchronization information contained therein to establish frame synchronization with the transmitting communication device. In this scenario, the frame synchronization takes approximately three hundred sixty (360) milliseconds (i.e., the duration of the superframe data 304 and the inbound talker SACCH data 306) and the audio information of superframe data 304 is lost.
If the FNE receives a request from another communication device to join the existing call after data components 302-314 have been transmitted from the transmitting communication device, then it must wait for the inbound talker SACCH data 322 to establish frame synchronization with the transmitting communication device. In this scenario, the frame synchronization takes approximately seven hundred twenty (720) milliseconds (i.e., the duration of superframes 316, 320, listener SACCH 318 and inbound talker SACCH 322) and the audio information of superframe data 320 is lost. An average call duration is typically around two (2) seconds. Therefore, loss of the audio information of superframe data 320 results in an approximately a thirty three percent (33%) total loss of audio information.